The kinetics and mechanism of oxidation of hydroxylamine by iron(iii)

Abstract

The kinetics of oxidation of hydroxylamine by iron(III) have been studied spectrophotometrically. The effects of pH, reactant and product concentrations led to two experimental rate expressions. The results obtained are consistent with a mechanism dependent on the relative iron(III)-to-hydroxylamine concentrations; an excess of iron(III) gives a stoichiometry of 2 ∶ 1 for the total reaction and N₂O as the oxidation product, whereas at equal amounts or excess of hydroxylamine the stoichiometry reduces to 1 ∶ 1 with N₂ as the main product . In the latter case the rate expression –d[Fe(III)]/dt = k[Fe(III)]²[NH₂OH]²/[Fe(II)]²[H⁺]⁴ is obtained. The consistent mechanism identifies two pre-equilibria and the reaction of two different nitrogen-containing intermediates as the rate-determining step depending on the reactant ratios. The proposed mechanism does also involve the metal hydroxide complex Fe(OH)²⁺, in agreement with previous results.